The invention provides a digital filter operating at intermediate frequency.
The information flows transmitted by microwave links may vary as whole multiples of the basic digital stream; this latter being 256 kbit/s. That is to say for example: 512, 1024, 2048, or 4096 Kbit/s etc..
Microwave transmission is optimized for each flow rate by a filter generally placed at an intermediate frequency (70 MHz very often) whose pass band is directly proportional to the data flow to be transmitted.
Depending on the data flows, filters are switched in, which may be of different kinds:
quartz or surface wave filters for small data flows,
localized constant filters for high data flows.
These techniques are unwieldly and costly in use.
The digital filters of the prior art have numerous advantages. They have a great flexibility in use due to their adaptivity and their possibility of performing non linear functions, from values of the programming coefficients;
Such filters however have drawback of comprising a structure such that delays Z.sup.-1 appear due to the operators inherent in the operation and structural delays due to adders.
The delays TO (time operator) Z.sup.-1 are related to the data flow : bit time. The structural delays Z+ appear as defects and degrade the operation of the filters.